The present invention generally relates to the detection and quantitative evaluation of seal leaks in fluid transport systems, particularly Emergency Shut Down valves, more commonly known as ESD valves used in such systems. ESD valves are in use in many commercial and industrial facilities, and in particular, are used extensively on all, or virtually all, of the approximately 2,000 off-shore oil and gas platforms around the world, to isolate the platform in the event of an emergency. The purpose of the present invention is to provide a means for more easily and accurately determining, both on-line and during shutdown, if each of the ESD valves on the platform has sufficiently small seal leaks to properly perform it's intended isolation function. The need to verify the ability of safety devices and their key components, such as ESD valves and their seals, to properly perform their safety function on off-shore platforms, has been made more apparent by the Piper Alpha disaster in the British sector of the North Sea, which led to the sinking of that platform in the late 1980's, with an accompanying significant loss of life.
Off-shore platforms contain many different valves of various designs and purposes. Even the ESD valves themselves, which typically number more than ten on a platform, may be of various designs. Thus, to be practical and effective, any methodology for assessing the integrity of ESD valve seals must be able to address in large measure, all the various designs. ESD valves are normally open in use, and closed only during shutdowns, either a normal production shutdown or an emergency shutdown. The majority of the ESD valves are ball valves, with only a small minority being gate valves. The ESD valves are usually installed in-line or in series with other valves that may also be closed off in a shutdown situation. However, the ESD valve is considered to be the valve of last resort--the valve that must provide isolation should all the other in-line valves fail to fully close, or leak. On the platform, the other in-line valves may serve to limit the differential pressure across the ESD valve following a shutdown, the duration of that differential pressure, or both, thereby precluding the most commonly used method for detecting a leak in a closed valve--high frequency acoustic leak detection which essentially listens for the sound of the leaking fluid. However, even in the presence of high differential pressure, such a technique would not be capable of detecting a seal leak in a valve where only an upstream seal or only a downstream seal leaks, but not both because such a valve will not actually leak. Nevertheless, with one seal leak already, such a valve would be in danger of becoming a valve that would soon leak through, and so the detection of a single leaky seal is almost as important as a valve where both the upstream and downstream seals leak.
At present, in order to identify and quantify ESD valves with seal leaks, off-shore platform operators sometimes resort to pressurizing the inner cavity of the valve during a shutdown period to see how well the valve holds the pressure. This is not a desirable method, as it adds costly time to an already costly production shutdown. Furthermore, it is not possible to perform such a test during production. What is needed is a non-intrusive test. A test that will yield information about seal leaks, in production as well as in shutdown situations, and will do so for all commonly used types of ESD valves.
This inventor is aware of other patents and application, disclosed and pending, which purport to at least partially accomplish that goal, and these will be addressed in the present application. But none of these, including the inventor's own pending patent application, come close to achieving the capability of the present invention--particularly in the area of seal leak quantification.
This inventor's pending patent application (frequently referred to in the text that follows as the previous invention, or the previous patent application), was filed on Jun. 12, 1998, and is based on Provisional Patent Application No. 60/055,728 filed Jun. 26, 1997, and Provisional Patent Application No. 60/060,590 filed Oct. 1, 1997 now U.S. Pat. Nos. 6,134,949 and 6,128,946. It involves the combined frequency analysis of simultaneously obtained dynamic signals from one pressure transducer sensing the dynamic cavity pressure in the valve, and another pressure transducer sensing either the upstream or downstream dynamic pressure. Specifically, the frequency analysis called for is one of either: the Frequency Response Function (FRF) sometimes known as the Transfer Function; or the Coherence Function; or a combination of the two. The FRF measures the magnitude and phase relationship that exists (if any) between two signals as a function of frequency, and the Coherence Function measures the degree to which a consistent relationship exists (i.e., one that is repeatable), also as a function of frequency. The described method can be used during production for those type of ball valves that do not have a bleed hole in the ball to equalize the inner cavity pressure to the line pressure. Without an equalizing bleed hole, high Coherence (particularly at low frequencies) can be attributed to a leak in one of the seals. That is because only a seal leak would allow flow noise from the fluid in the pipe to enter the inner cavity, in the same way that a door to a room may be cracked open a bit to allow noise from the hallway to enter the room. The Coherence verifies that it's not just any noise that's entered the inner cavity, but the same noise that's in the pipe. When a pressure equalizing bleed hole exists in the ball, the method won't work because flow noise from the pipe gets into the inner cavity through the bleed hole, and causes high Coherence (nearly equal to 1.0), independent of whether or not a seal leak is present.
With no flow, such as when the valve is closed during a production shutdown, there is no flow noise in the pipe to make the above method of the previous invention work. For that reason, the aforementioned previous patent application also describes the use of an external sound source which injects either a single low-frequency sound, or highly repetitive clicks at some constant low frequency rate (the latter naturally including harmonics of that low frequency rate as well) into the inner cavity of the valve and checks for evidence of the sound upstream or downstream. The same frequency analyses (FRF or Coherence) are performed encompassing the frequency of the source (and if applicable, its harmonics) to determine whether a seal leak is present. As will be shown herein, the previous invention (although a step beyond prior methods) has many shortcomings.
The present invention eliminates many of the shortcomings of the previous invention, by recognizing that: 1) time, rather than frequency, is a more effective domain for analysis, especially when leak quantification is the goal; 2) the combination of a unique time domain analysis method with the use of a random pulse generator enables production and shutdown situations to be treated alike, more simply, and more accurately; and 3) ball valves with bleed holes can be addressed in production situations using the procedures of the new invention.
The present invention will help off-shore oil and gas producers achieve two main goals: To improve safety on off-shore platforms by properly identifying ESD valves with unacceptable seal leaks. And to reduce costly unnecessary maintenance by properly identifying ESD valves with acceptable seal leaks.